1. Field of the Invention
The present invention relates to a charge pump and the method for operating the same, and more particularly, to a charge pump which has limited voltage variation.
2. Description of the Related Art
It is a clear trend that semiconductor memory devices are becoming more highly integrated and low operating supply voltages are being widely used. However, even memory devices that operate at a low voltage still need high voltage power supply for certain internal circuits and operations such as driving bit lines and word lines. For such a purpose, a variety of charge pumps for generating high voltage have been developed.
FIG. 1 shows a traditional charge pump 10, which is driven by a constant frequency clock OSC. However, there is an excess of charge pump output and power consumption in a high-voltage state.
U.S. Pat. No. 6,490,220 discloses a multiple core charge pump including a plurality of switches disposed between the taps of a delay chain and the individual charge pump cores. The charge pump is divided into five charge pump cores to avoid a loud noise due to simultaneous conduction. For this purpose, delay chains are used to separate the turn-on time of each charge pump core. However, the delay chains are highly dependent on the ambient temperature and operating voltage. If there is any unusual variation in the ambient temperature and operating voltage, the charge pump will fail to achieve its original purpose. Or the delay chains can be made insensitive to temperature, process and voltage. But this may need a complex design.